Lap



Dec. 24, 1940. AN E PYL 2,226,506

LAP

Filed 001;. 2, 1940 EDWARD VAN DER PYL Patented Dec. 24, 1940 LAP Edward Van der Pyl, Holden, Mass., assignor to Norton Company, Worcester, Mass., a. corporation of Massachusetts Application October 2, 1940, Serial No. 359,420

6 Claims.

The'invention relates to laps and other abrasive articles, particularly for grinding lenses.

One object of the invention is to provide a lens lap of long life. Another object of the invention is to provide a construction for a lap for lens grinding whereby substantially all of the abrasive material will be used during the life of the lap. Another object of the invention is to provide a diamond lap of spherical shape with the abrasive so distributed that little thereof is left when the lap is worn out. Other objects will be in part obvious or in part pointed out hereinafter.

The invention accordingly consists in the features of. construction, combinations of elements and arrangements of parts, all as will be illustratively described herein, and the scope of the application of which will be indicated in the following claims.

In the accompanying drawing showing two of many possible embodiments of the mechanical features of this invention,

1 Figure 1 is a plan view of a metal back for the P;

Figure 2 is a cross-sectional view through a mold, showing the first stage of a molding operation;

Figure 3-is a cross-sectional view through the same mold, showing a later stage in the molding operation;

Figure 4 is a side elevation of a positive lap constructed according to the invention, the center part being broken away, showing the lap in cross section; and

Figure 5 is a view, partly in side elevation and partly in cross section, of a negative lap constructed in accordance with the invention.

Referring first to Figure l, I provide a metal back In shaped into partial spherical form. That is to say, the metal back III was originally a thin disk of metal; it-was pressed to partial spherical sheet form, as illustrated by comparison of Figures 1, 2 and 3. It can be described as a uniform thickness shell with similar spherical surfaces, one concave, the other convex, the surfaces being bounded by small circles (of the sphere). Also since the spherical surface to either side ofa plane through a sphere is referred to as a one base zone, the metal back II! can be referred to as a shell bounded by equal one base zones, or a one base zone shell, or simply a zonal shell.

Brass is a very satisfactory metal for the back ill but other metals may be used, such as copper, bronze, aluminum, and various kinds of steel and iron. With a lathe tool or the like, I form a number of circular grooves II in the back HI.

Preferably these grooves are concentric, their center being the center of thedisk from which the back ID was formed. On the other hand, it is entirely feasible to form a continuous spiral groove with equally good results. As shown, these 5 grooves ll undercut the material of the back I0 so that the abrasive-bond mixture will interlock therewith. Also, the grooves II are made with a tool inclined to the sphere axis (preferably ,pointing towards it) rather than parallel to the m sphere axis, which also gives an interlocking construction.

As shown in Figure 3, I provide suitable mold equipment, such as a moldring II, a bottom plate 13, and a top plate H. The ring I! may 15 have cylindrical outer and inne'r'surfaces and plane endsurfaces, as shown. The bottom plate I3 is a right circular cylinder modified by a one base zone at one end, The top plate H is likewise a right circular cylinder modified by a one 20 base zone. However, the spherical zone 0 the bottom plate I3 is convex while the spherical one of the top plate It is concave.

I provide a suitable mixture 20 of diamond abrasive and bond. For bond I may use powdered 25 A stage resin, for example phenol formaldehyde, or any one of the polyhedric alcohol-polybasic acid resins, for example glycerol phthalate, or aniline formaldehyde condensed in the presence of a strong acid with or without modifying in- 30 gradients, or methyl methacrylate preferably hardened with a suitable cross-linking agent, for exampleup to 30% of methacrylic acid or other suitable synthetic resin. On the other hand, I may also use metal powders to form, after pres- 35 sing and 'sintering, a metal bond. Preferable metal bonds are disclosed in my Reissue Letters Patent No. 21,165. Excellent results in actual practice are achieved using of the order of 20% tin, the remainder copper. The tin and copper 4 can be used in the form'of finely divided powders which are mixed in a suitable mixing machine with each other and then with the diamond abrasive.

Referring now to Figure 2, I place the metal back Ill upon the bottom plate 13 with the ring I2 surrounding the back it and bottom I3. I then introduce the abrasive-bond mixture 2F c top of the back l0. With a shaped scraper 2| which is a piece of sheet metal the bottom side of which is a secanted circle, I level the abrasivei bond mixture 20 to form a one base zone top surface.

A particular feature ofthe invention resides in formation of a final lap wherein the spherical radius of the abrasive surface is different from the spherical radius of the underlying surface of the back platefllll. In the case of a positive lap, the spherical radius ofthe abrasive surface will be geeater than that of the underlying zone B of the back plate M. In the case of a negative lap, the spherical radius of the abrasive surface will be less than that of the underlying zone of the back. In each case, in the final article, the abrasive layer 25 is thicker at the periphery than in the center. In fact, the molding operation is carried on so as to produce an abrasive coating on the metal back plate ll) of a substantial finite thickness at the periphery, which thickness decreases by uniform amounts as the center is apzero at the center. Thus the two zones are practically tangent and they have a common polar axis identical with the axis of the mold but their spherical radii are of different lengths, as stated. 20 Having thus distributed the abrasive-bond mixture 20 in the mold upon theback plate III by rotating a shaped scraper 2|, I apply the top plate H. The mixture in the mold is then sublected to a pressure of usually several tons per square inch to pack the mixture onto thebacking I0, interlocking it with the grooves I I. The mold is then stripped and the article placed in an oven or furnace to mature or sinter the bond. In the case'of phenol formaldehyde, a temperature of 175 C. over four or five hours will mature the bond to the irreversible C stage. In the case of a metal bond, which is disclosed in my above numbered reissue patent, that is to say, the copper-tin bond, a hydrogen atmosphere furnace is preferably used and a sintering temperature of 600 C. may be employed. For other bonds, the technique is that of maturing the bond according to methods known in the arts.

In order to attach the lap to a lens lapping ma- 40 chine, it is desirable toform a central hole 26 which may be countersunk, as shown. For that reason, it is unnecessary to extend the grooves l I quite to the center of the back plate I. This central hole 26 can best be formed after the 45 abrasive material has been molded in place. Since all of the zonal surfaces are now between parallel cutting planes, they may be referred to as two base zones, but as the cutting of the central hole may in some cases be dispensed with, I 59 may refer to the surfaces generically as zones.

Figure 5 discloses a back plate Illa with grooves Ila made with a lathe tool facing away from the polar axis of the zone surface, giving a good interlocking action for a negative lap, that is, a 65 concave surface. The back Illa is the same as the back l excepting that the grooves are formed on the opposite side and incline differently, as stated. The abrasive coating 25a has an outer zone surface which has a smaller radius than 00 that of the concave zone face of the back 10a.

However, in the case of the negative lap of Figure as well as in the case of the positive lap of Figure 4, the abrasive coating is thickest at the periphery of the lap and diminishes toward the 65 center. The lap of Figure 5 has a countersunk central hole 26a.

To distinguish between zonal surfaces, I refer to the surfaces of the base and the abrasive portion which contact each other as the inside sur- 70 faces, the others being the free or outside surfaces.

In the use of optical laps, either the lap or the work piece is revolved. The greatest relative motion is at the periphery and the relative motion 75 diminishes toward the center. In fact, it may preached and (theoretically) drops to practically of laps will be kept on hand for making various shapes of lenses, due account can be made of the change in curvature and the lap when new may be useful for lapping lenses of a certain diopter, while later on it may be useful for lapping lenses of different diopters.

The advantage of using a zonal shell for the base instead of using a solid zone is that duplicates can quickly be made by a metal pressing operation. That is to say, a piece of sheet brass can be pressed to zonal shell shape between two dies shaped like the mold between. plate I3 and top plate It except that they should have the same spherical radius. Furthermore, with the zonal shell base, the final article is lighter.

It will thus be seen that there has been provided by this invention an article and a method in which the various objects hereinabove set forth together with many thoroughly practical advantages are successfully achieved. As various possible embodiments might be made of the mechanical features of the above invention and as the art herein described might be varied in various parts, all without departing from the scope of the invention, it is to be understood that all matter hereinbefore set forth or shown in the accompanying drawing is to be interpreted as illustrative and not in a limiting sense.

I claim:

1. A lap comprising an abrasive layer bounded by zones with a common polar axis, the spherical radii of the zones being different so that the abrasive layer is thicker at the periphery and diminishes in thickness as the pole of the zones is approached, and a non-abrasive backing integral with the abrasive layer, the abrasive layer constituting comminuted abrasive material and a bond therefor.

2. A lap according to claim 1 in which the base is a zonal shell.

' 3. A lap according to claim 1 in which the abrasive layer and the baseare interlocked by grooves. in the zonal surface of the base, the grooves being approximately circles concentric with each other, the center being the polar axis of the zone and the grooves having a side wall which is not parallel to the polar axis of the zone.

4. A lap comprising a zonal shell of metal having one zonal surface modified by substantially concentric grooves whose center is the polar axis of the zonal shell, the grooves being undercut as by being formed by rotating the zonal shell on its polar axis and cutting the grooves with a tool inclined to said axis, and a layer of abrasive material constituting comminuted abrasive bonded together, integrally secured to the zonal shell with said abrasive material in said grooves, and the free abrasive surface of the abrasive material being a zone of different spherical radius than the zones of the zonal shell, the abrasive material being thicker at the periphery than towards the center of the lap.

5. A lap according to claim 4 in which the inside zonal surface of the shell and the outside zonal surface of the abrasive material are tangent at the poles.

6. Method of making an optical lap comprising rotating a metal zonal shell on its polar axis, cutting grooves in said zonal shell with a lathe tool inclined to the polar axis, placing-the zonal shell in a mold, distributing over the grooved zone of the zonal shell a mixture of comminuted abrasive and bond, spreading the mixture with a spreader whose spreading edge is asecanted circle by rotating the spreader, pressing the mixture with a mold top plate which has a bottom zonal surface with a spherical radius different from that of the zonal radius. of the zonal sheiland forming a thicker layer of abrasive and bond at the periphery of the zonal shell, then maturing the bond to form an integral article.

- EDWARD VAN DER PYL. 

